docs: def url(self):
          """The url for this collection."""
          if self.parent is None:
              # TODO: differing API Versions?
              pieces = [self.client.base_url, 'api', 'atlas', 'v2']
          else:
              pieces = [self.parent.url]
  
          pieces.append(self.model_class.path)
          return '/'.join(pieces)